The noscript element must contain only
text, except that the text must be such that running the following
algorithm results in a conforming document with no noscript elements and no script
elements, and such that no step in the algorithm causes an HTML
parser to flag a parse
error:

The noscript element is only effective in
the HTML syntax, it has no effect in
the
XHTML syntax. This is because the way it works is by
essentially "turning off" the parser when scripts are enabled, so
that the contents of the element are treated as pure text and not
as real elements. XML does not define a mechanism by which to do
this.

In the following example, a noscript element is used to provide
fallback for a script.

When script is disabled, a button appears to do the calculation
on the server side. When script is enabled, the value is computed
on-the-fly instead.

The noscript element is a blunt instrument.
Sometimes, scripts might be enabled, but for some reason the page's
script might fail. For this reason, it's generally better to avoid
using noscript, and to instead design the
script to change the page from being a scriptless page to a
scripted page on the fly, as in the next example: